IEA Solar Heating and Cooling Programm - NachhaltigWirtschaften.at

IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask C Report, Date: 13.01.2009
2.3 Water treatment
The makeup water is fresh water added to the cooling towers to replace evaporation, blow
down, and drift losses. The amount and chemistry of makeup water added directly affects
the quality of water in the systems.
The relationship between blow down water quality and make-up water quality can be
expressed as a concentration ratio. This concentration ratio is typically between 2 and 5
(VDMA 24649), which means that the salt concentration in the circulating water is 2 to 5
times higher than in the makeup water. Limits for the concentration of different minerals are
usually given in the operating manuals of the cooling tower manufacturer.
As the concentration of salts increase the water may not be able to hold the minerals in
solution and they can precipitate out as mineral solids and cause fouling and heat exchange
problems in the cooling tower.
Beyond the necessary limitation of the mineral content water treatment might be required to
avoid scale, corrosion and growing micro organism.
2.4 Aerosols in the Waste Air
As discussed a small portion of the cooling water is lost in form of mist carried out of the
tower with the waste air. In Order to limit these aerosols in the waste air drift eliminator are
installed in the air outlet. The effectiveness of these drift eliminator varies in a far range
between 0.0005 and 0.1% of the circulation cooling water flow rate is released to the waste
air (Aquaprox, 2007). This means that the drift eliminator affects the aerosol discharge of the
cooling tower and with that also the release of micro-organism very much.
However, it must be assumed that some droplets are within the critical size for human intake
of 5 micrometer or smaller. Larger droplets leaving the cooling tower may be reduced to 5
micrometer or less by evaporation (ASHRAE Guideline 12-2000).
Even the best drift eliminators do not eliminate aerosols entirely. Thus it cannot be the “stand
alone“ preventive measure but high efficiency drift eliminator are able to reduce the release
of micro-organism significantly. Furthermore the evaporative cooling equipment should be
positioned such that it is away from occupied areas or where drift can enter directly into the
windows or air intakes of buildings in the vicinity of the installation. The prevailing wind
direction should be taken into account wherever possible EUROVENT 9/5 (2002)
2.5 Electricity Consumption
The electricity consumption of a cooling tower is mainly determined by the electricity demand
of the pumps and fans. As discussed, compared to the cooling capacity of a thermally driven
heat pump the cooling capacity of the heat rejection system is approx. 2 to 3 times bigger.
This leads to a significant electricity consumption of the heat rejection system in solar
cooling applications. In order to reduce this demand variable speed controlled pumps and
fans can be considered.
2.6 Anti freeze
Solar cooling applications are not in operation during winter season and cooling towers will
be drained during this period of time thus no freezing problems occur. However, if there is a
risk of freezing appropriate counter measures are needed e.g. electric immersion heater in
the collection basin.
2.7 Part Load Operation
Typically the speed of the fans can be controlled in two steps. Alternatively it is possible to
equip the fan with frequency control in order to control the required cooling capacity.
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